Many ICs are made up of millions of interconnected devices, such as transistors, resistors, capacitors, and diodes, on a single chip of semiconductor substrate. CMOS circuits and fabrication technology are commonly used in complex ICs. CMOS circuits use PMOS and NMOS devices to implement functions such as logic and input/output (“I/O”) blocks.
An I/O block is a circuit in an IC that receives or sends data from or to other ICs. Signals can be differential (i.e., a HI/LOW or LOW/HI signal is simultaneously provided on differential I/O pins) or single-ended (i.e., either a HI signal or a LOW signal is provided on a single pin). In some ICs I/O blocks can operate on either differential signals or on single-ended signals. When the I/O block operates in differential mode, an on-die differential termination is provided to terminate the differential path with an appropriate impedance (load resistor). When the I/O block operates in single ended mode, the differential termination is turned off.
However, if an undershoot occurs when an I/O block is being operated in single-ended mode, NMOS transistors can turn on and conduct if the undershoot exceeds the threshold voltage of the transistor(s). Undershoot can occur due to many factors, such as switch bounce, reflections from improperly terminated transmission lines, or inductive effects. Undershoot can cause leakage current to flow between the differential pins of an I/O block when it is operated in single-ended mode. For example, if the single-ended input pin is in transition from a HI signal to a LO signal, the falling edge of the pulse can undershoot and turn on transistors, allowing leakage current.